1. Field of the Invention
The present invention relates to a switch control in a scanning LED display, and more particularly, to eliminate ghost images of the scanning LED display due to the switching of the scan lines in the LED display.
2. Description of Related Art
FIG. 1 illustrates a driving circuit in a conventional LED display using scan lines. There is a switch SW(N) for controlling a corresponding scan line N, wherein one end of the SW(N) is connected to anodes of the LED(s) in the scan line (N), and another end of the SW(N) is connected to a power supply voltage, such as Vsupply+. The SW(N) is controlled by an enable signal S(N), wherein when the S(N) is enabled, the SW(N) is turned on, or closed; otherwise, SW(N) is turned off or opened.
As shown on FIG. 1, when the SW(1) is turned on and the voltage of the anodes of the LED(S) in scan line (1) will be charged to Vs1; when the SW(2) is turn on and the voltage of the anodes of the LED(S) in scan line (2) will be charged to Vs2; when the SW(2) is turn on and the voltage of the anodes of the LED(S) in scan line (3) will be charged Vs3; and when the SW(4) is turn on and the voltage of the anodes of the LED(s) in scan line(4) will be charged to Vs4. Currents I1, I2, I3 and I4 flowing through each row of the LED(s) of the scan lines can be conditioned by current sources connected to cathodes of the LED(s) of the scan lines, respectively, wherein a bias voltage is applied at cathodes of LED(s) in each row respectively. Each of the scan line will be scanned by activating the corresponding enable signal of the scan line in a pre-determined order, for example, S1→S2→S3→S4. In each scan line(N), there is a corresponding parasitic capacitance Cp1, Cp2, Cp3, Cp4, coupled from the anodes of the LED(s) of the scan lines to the ground voltage, respectively, as shown in FIG. 1.
FIG. 2A shows a 4×4 LED display using scan lines. Assume that there is an image in which the diagonal LED(s), D11, D22, D33 and D44, are conditioned to emit light, and other LED(s) are conditioned to be turned off and remains dark. However, in addition to the diagonal LED(s), D11, D22, D33 and D44 that are configured to be turned on in the image, some other LED(s) that are supposed to remain dark are actually turned on for a short time period and emit light briefly, which is not supposed to happen in the original image. When the scan line is switched from scan line 1 to scan line 2, that is, when S1 goes low to turn off the SW(1) and S2 goes high to turn on the SW(2), the charge stored in the parasitic capacitance Cp1 has no conductive path to discharge when the SW(1) is turned off; therefore, the voltage at anodes of the LED(s) of the scan line 1 will be kept at voltage VS1. Because D22 is turned on, the current I2 will be conducted by pulling the voltage at the cathode of the D22 to a low voltage level to allow the D22 to emit light. Since the cathodes of the D12 and D22 are connected together, D12 will be turned on due to the fact that the anode voltage of the D12 is kept at the voltage VS1 by the parasitic capacitance Cp1. At this time, the voltage VS1 will be discharged quickly through the D12 as well. As a result, the D12 will be turned on only for a very short time period and therefore generates visual effects, which can be evidenced by a glitch occurred to the signal ID12 in FIG. 2B. Such visual effects will be contributed by the LED(s) D23, D34, and D41 as well, which can be evidenced by a glitch in each of the signal ID23, ID34, and ID41 in FIG. 2B.